The present invention relates to a method of stretching plastics material having a pattern of holes or mesh openings to form molecularly orientated strands from zones (called "strand-forming zones" herein) of the plastics material. In some cases, the whole of the plastics material can be stretched out to form orientated strands. The holes may not penetrate right through the plastics material or the mesh openings may contain membranes, if desired.
The method of the invention has been found to be generally applicable, for instance to stretching integrally extruded mesh structures, e.g. those produced as described in GB No. 836 555, GB No. 969 655 or GB No. 1 250 478. The invention is more particularly applicable to methods of producing integral plastics material mesh structures by stretching starting materials having a pattern of holes which can have been formed for instance by punching. Examples of such methods are described in for instance GB No. 2 031 833A, GB No. 2 034 240A, GB No. 2 035 191B, GB No. 2 059 866A, GB No. 2 073 090B, GB No. 2 096 531A, GB No. 2 108 896A, and a U.S. application by the same Applicant filed July 1, 1983 under the title "Mesh structure and laminate made therewith".
Nonetheless, the invention was made in connection with producing biaxially-stretched materials in accordance with GB No. 2 035 191B, and will mainly be described in relation to GB No. 2 035 191B, the mesh structure produced being called "biax Square Mesh" herein for convenience.
The standard way of manufacturing biax Square Mesh is to MD (machine, i.e. longitudinal, direction) stretch using nip rolls, relax 5 to 10%, TD (transverse direction) stretch on a stenter, and relax 5 to 10%. On the stenter, there will be a large number of mesh openings across the width. It was observed that the TD stretching of the mesh openings or TD strands was not regular. As most of the initial increase in width was due to only a few strands stretching, there was very rapid stretching of those strands. In extreme cases, the strands could fibrillate and/or rupture. However, apart from the uneven appearance of the final product, less extreme cases could still cause grave disadvantages; on the first (MD) stretch, a slightly different penetration into the bar or a slightly different amount of penetration across the bar could greatly change the junction formed after the second (TD) stretch; if the increased penetration caused the junction to stretch before the strands in the second stretch, a much weaker and radically different junction would be formed.
Similar differences occurred when the second stretch was MD; as the stretch length cannot be very short in practice, there is always a number of aligned strands stretching at the same time. Such differences, though not so marked, were also apparent when making uniaxially-stretched structures.
In general terms, more difficulties were experienced with polypropylene (PP) than with high density polyethylene (HDPE), but the differences were apparent with HDPE.